High-frequency noise in power transmission is detrimental to the operation of a semiconductor circuit. Unfortunately, it is unavoidable due to the external sources of radiation and the nature of semiconductor devices. Besides utilizing a radiation shield for isolating external noise sources, decoupling capacitors have been widely employed to reduce noise or noise transmission over circuit or system power line. By introducing a low impedance path between power transmission line and system ground, decoupling capacitor attenuates high frequency noise in power line. Not only as a discrete component, decoupling capacitor is also integrated into large semiconductor chips. Typically, in a large integrated circuit different function blocks have their own integrated decoupling capacitors to minimize the noise influence over power line.
As decoupling capacitors are typically biased at full power supply voltage (Vdd) during circuit operation, their reliability or wearout has always been a concern. Particularly, with the introduction of deep-trench decoupling capacitor embedded in the silicon substrate, it is expected that its reliability becomes more critical due to the corners around the trench bottom as the weakest links for dielectric breakdown.
The wearout of decoupling capacitors is due to the degradation or breakdown of its dielectric materials, including hard-breakdown and soft-breakdown. The hard-breakdown of the capacitor causes electric short and thus results in failure of the whole circuit. Before hard-breakdown, the capacitor dielectric material always has to go through the soft-breakdown regime, which results in gradual increase in leakage current and noise level even though the capacitor still remains functional. The extra noises generated in the soft-breakdown regime spread over a wide frequency range, from a few Hz up to hundred Hz range, for example, as shown in the graph of FIG. 1 depicting an example plot 10 of l/f noise spectra of gate current 12 both before soft breakdown and gate current 15 after soft breakdown of a 2 nm oxide of the decoupling capacitor. As shown, the noise level increases by 10,000× after soft breakdown that is detectable before catastrophic “hard breakdown”. The increase of the noise caused by dielectric soft-breakdown has been shown in the reference entitled “Ultra-thin gate dielectrics: they break down, but do they fail?” by B. E. Weir et al. on International Electron Devices Meeting, 1997. Technical Digest., 7-10 Dec. 1997 pp: 73-76.
As illustrated in FIGS. 2A-2C, it is noted that a fresh decoupling capacitor 25A effectively removes high-frequency noise signals 27 from the supplied power line 30 leading to semiconductor circuitry 50; and, slightly degraded decoupling capacitors 25B in soft-breakdown regime are still functional in filtering high-frequency noise 27′, since the low-frequency noise 28 resulted from soft-breakdown does not impact the semiconductor circuitry 50 significantly (see FIG. 2B). However, failed decoupling capacitors (decap), such as decap 25C that reach hard-breakdown regime cause catastrophic circuit failure of semiconductor circuitry 50 (see FIG. 2C) as power signals and high frequency noise 29 are shorted to ground 19 through the decap.
By measuring power line low frequency noise caused by dielectric soft-breakdown, the decoupling capacitor wearout can be monitored. However, there is no current technique or solution available to accurately sense the noise level in soft-breakdown period of the decoupling capacitor.
There are some teachings relating to power line noise sensing, such as described in U.S. Pat. No. 7,355,429 entitled “On-chip Power Supply Noise Detector,” U.S. Pat. No. 7,355,435 entitled “On-chip Detection of Power Supply Vulnerability,” U.S. Pat. No. 7,301,320 entitled “On-chip High Frequency Power Supply Noise Sensor,” and U.S. Pat. No. 6,605,929 entitled “Power Supply Noise Sensor.” However, all this prior art teaches measuring the voltage overshoots and/or undershoots, which do not represent the real noises of the power supply line. Hence, they are not suitable as representing wearout information of decoupling capacitors.
In addition, there are some prior arts related to decoupling capacitors, such as U.S. Pat. No. 7,227,211 entitled “Decoupling Capacitors and Semiconductor Integrated Circuit,” and U.S. Pat. No. 6,011,419 entitled “Decoupling Scheme for Mixed Voltage Integrated Circuits.” Again, none of these references deal with decoupling capacitor reliability or wearout issues.
It would be highly desirable to provide a system, method and circuit with ability to detect early signals before catastrophic capacitor failure, to pin-point worn-out capacitor(s) and to disable it(them), and avoiding impact from capacitor breakdown failure.
It would be further highly desirable to provide a system, method and circuit that provides for decoupling capacitor redundancy.